The objective of this project is to gain further insight into mechanisms and energetics of Ca2 ion activation of striated muscle contraction. The following rationale will be employed. If a muscle is stretched to a point of vanishing myofilament overlap and stimulated, net energy liberated is related to ATP hydrolyzed. It seems likely that this ATP is split by the sarcoplasmic reticulum to resequester Ca2 ion that was released during muscle activation. Assuming that all Ca2 ion released must be actively reaccumulated again, then net energy liberation from stretched muscles indirectly monitors amount of Ca2 ion released with stimulation. Thus alterations in time course and amount of energy liberation associated with Ca2 ion cycling may reflect alterations in Ca2 ion release, action and reaccumulation. Energy liberation during isometric contractions in isolated striated muscles will be monitored with sensitive thermopiles. Aims are designed to clarify: (1) relationships of activation energetics to subsequent muscle force development and (2) source(s) of time course of energy liberation associated with activation. Aims include investigation of effects of the following conditions/pertubations on activation energetics: (1) previous muscle activity in the form of twitches, brief tetani, and fatiguing tetani, (2) chemical agents that are established or putative modifiers of excitation - contraction coupling, (3) muscle preparations with different contraction times, (4) muscle preparations possessing thick filament rather than thin filament regulation of contraction, (5) muscle pH and temperature, and (6) external work done on muscle during activation. Aims will be achieved by measurement and computer analysis of amount and kinetics of energy liberation associated with the cycling of Ca2 ion.